1. Field of the Invention
This invention relates to a plug for use in wellbore operations and an apparatus for launching said plug.
2. Description of the Related Art
During the construction of an oil or gas well a hole is bored in the ground. A string of casing is lowered down the hole and the annular space between the casing and the wellbore filled with cement.
In order to inhibit the cement mixing freely with other wellbore fluids it is normal to use two devices which are referred to as "cementing plugs". When it is desired to cement the casing in place a first (bottom) cementing plug is released into the casing. The cement is then pumped into the casing and pushes the bottom plug downwardly. When the desired volume of cement has been introduced into the casing a top plug is introduced into the casing and pumped downwardly by a convenient fluid, typically sea water or circulating fluid. The cement then travels down the casing trapped between the bottom and top plugs. When the bottom plug reaches the bottom of the casing it lands on a one way valve known as a float collar. Pressure is applied to the top plug until a bursting disk in the bottom plug ruptures and allows the cement to be pushed through a flow bore in the bottom plug, through the float collar and upwardly into the annular space between the casing and the bore. The description thus far relates to normal operation. However, occasionally the bottom plug and the top plug are released simultaneously with the result that a large column of cement is pumped down the casing on top of the top plug. Since the top plug is designed to withstand high pressures it will not rupture and consequently the cement sets and has to be drilled out.
A large number of devices have been designed for preventing inadvertent simultaneous release of the bottom plug and top plugs and those which have been designed for use at the surface of a land well are generally satisfactory. However, the problem which does remain is when a casing is extended by a liner and it is necessary to cement the liner in position. In this case the bottom and top plugs have to be released remotely from the surface. Although there are a number of apparatus intended for inhibiting simultaneously release of the top and bottom plugs in these circumstances it is still not unknown for the plugs to be released simultaneously with expensive consequences.